1. Field of the Invention
The present invention relates to electronic circuits. In particular, the present invention relates to power amplifiers.
2. Description of the Prior Art
In general, electronic power amplifier circuits have been produced in the prior art. The following prior art patents are representative of known electronic power amplifier circuits:
1. U.S. Pat. No. 4,719,431 issued to Karsten on Jan. 12, 1988 for "Audio Power Amplifier".
2. U.S. Pat. No. 4,165,494 issued to Becker on Aug. 21, 1979 for "Bi-State Linear Amplifier".
3. U.S. Pat. No. 4,167,708 issued to Goto on Sep. 11, 1979 for "Transistor Amplifier".
4. U.S. Pat. No. 4,135,162 issued to Takahashi on Jan. 16, 1979 for "Power Amplifier Circuits".
5. U.S. Pat. No. 4,424,493 issued to Schroeder on Jan. 3, 1984 for "Cross-Coupled Complementary Power Amplifier".
6. U.S. Pat. No. 4,540,951 issued to Ozawa et al. on Sep. 10, 1985 for "Amplifier Circuit".
7. U.S. Pat. No. 4,404,528 issued to Yamaguchi on Sep. 13, 1983 for "Output Amplifier".
8. U.S. Pat. No. 4,237,425 issued to Spiegel on Dec. 2, 1980 for "Automatic Bias Adjusting Circuit".
9. U.S. Pat. No. 4,220,930 issued to Ahmed on Sep. 2, 1980 for "Quasi-Linear Amplifier With Feedback-Controlled Idling Currents".
10. U.S. Pat. No. 4,031,482 issued to Tsurushima on Jun. 21, 1977 for "Bias Circuit For FET".
11. U.S. Pat. No. 4,454,479 issued to Spires on Jun. 12, 1984 for "Operational Amplifier With Improved Output Capability".
12. U.S. Pat. No. 4,342,966 issued to Tamura on Aug. 3, 1982 for "Power Amplifier Circuitry".
13. U.S. Pat. No. 3,912,981 issued to Tsurushima on Oct. 14, 1975 for "Protective Circuit For Field Effect Transistor Amplifier".
14. U.S. Pat. No. 4,057,764 issued to Yokoyama on Nov. 8, 1977 for "Amplifier".
15. U.S. Pat. No. 4,178,559 issued to Nichols on Dec. 11, 1979 for "Amplifier Distortion Reduction Apparatus".
16. U.S. Pat. No. 4,723,111 issued to Verhoeven et al. on Feb. 2, 1988 for "Amplifier Arrangement".
U.S. Pat. No. 4,719,431 issued to Karsten discloses an audio power amplifier which comprises a pair of input terminals, a pair of output terminals, a pair of matched output tubes and a pair of output power supplies, and all of which are connected to form a bridge circuit in the power amplifier circuitry. This power amplifier is capable of producing high output power levels over a wide frequency range and uses fairly few components. However, it is a double-ended electron tube amplifier.
U.S. Pat. No. 4,165,494 issued to Becker discloses a bi-state linear amplifier which is particularly adapted for use in modern computer data bus systems. This linear amplifier is selectively opened or closed in response to a separate input control signal. It uses diodes in its input and output circuits but the internal opening or closing status of the amplifier is not determined by the diode means, but rather by the separate input control signal.
U.S. Pat. No. 4,167,708 issued to Goto discloses a transistor amplifier which comprises an input stage circuit and an output stage circuit direct coupled to the input stage circuit. The input stage circuit includes the complementary symmetry transistors connected to the constant current circuit. The output stage circuit includes the complementary symmetry push-pull transistor amplifier. It uses a constant current circuit which comprises the field effect transistor (FET).
U.S. Pat. No. 4,135,162 issued to Takahashi discloses a power amplifier circuit which comprises two input differential amplifier circuits, one connecting complimentary symmetry circuit and two output complementary symmetry push-pull circuits. It uses a diode and a phrase-inverting transistor to form a single current mirror circuit between the second differential amplifier circuit and the complimentary symmetry circuit, and a series of diodes in the output complimentary symmetry push-pull circuit to form a biassing circuit. However, it uses FET's for first and second differential amplifiers and does not have a diode-transistor circuit to adjust the working point of the transistors.
U.S. Pat. No. 4,424,493 issued to Schroeder discloses a cross-coupled complementary power amplifier circuit which also uses a diode for the biassing circuit, but neither has a diode-transistor circuit to adjust the working point of the transistors.
U.S. Pat. No. 4,540,951 issued to Ozawa et al. discloses an amplifier circuit which comprises several circuits including a current mirror circuit and a DC feed-back circuit to reduce non-linear distortion in the output signal caused by the base-emitter voltages of the transistors. It uses a circuit to maintain the current constant. However, the working point of the transistors in the output circuit is not adjustable.
U.S. Pat. No. 4,404,528 issued to Yamaguchi discloses an output amplifier which has four circuits comprising biasing transistors as well as output amplifying transistors. The attention of the circuit is focused on reducing the output waveform distortions and changes in the idling current corresponding to changes in the bias current.
U.S. Pat. No. 4,237,425 issued to Spiegel discloses an automatic bias adjusting circuit which is a double-ended bias adjusting circuit for a push-pull amplifier and also focused on the bias current stability.
U.S. Pat. No. 4,220,930 issued to Ahmed discloses a quasilinear amplifier with feedback-controlled idling currents which provides a circuit for establishing the idling currents in the Class AB push-pull output transistor amplifier stages.
U.S. Pat. No. 4,031,482 issued to Tsurushima discloses a bias circuit for FET which provides a circuit for compensating DC bias current changes in the drain of a field effect transistor (FET) caused by power supply fluctuations.
U.S. Pat. No. 4,454,479 issued to Spires discloses an operational amplifier with improved output capability which is focused on providing both maximum output voltage swing and high output current capability.
U.S. Pat. No. 4,342,966 issued to Tamura discloses another power amplifier circuitry which uses two internal batteries in the output stage circuit for passing the base currents of the output transistors to prevent the switching distortion.
U.S. Pat. No. 3,912,981 issued to Tsurushima discloses a protective circuit for field effect transistor (FET) amplifier which provides an improved protective circuit for an FET amplifier and is particularly adapted for use with an amplifier using at least one FET for amplifying purposes.
U.S. Pat. No. 4,057,764 issued to Yokoyama discloses an amplifier which is also an FET amplifier.
U.S. Pat. No. 4,178,559 issued to Nichols discloses an amplifier distortion reduction apparatus which uses diodes inserted in series with resistors in the output complimentary symmetry push-pull circuit serving as parallel impedance function elements so enough compensation can be obtained to reduce the distortion through the output amplifier circuit. However it reduces the nonlinearity by means of reducing the nonlinearity components of the output signal but does not initially cause the transistors to work at the most linear portion of their current-voltage characteristics.
U.S. Pat. No. 4,723,111 issued to Verhoeven et al. discloses an amplifier arrangement which is a particular configuration for output amplifiers operating in Class AB.
Overall, there are numerous power amplifier circuits. Some of them are for Class AB amplifiers, and some for FET amplifiers. However, none of the circuits of a Class B complimentary symmetry push-pull transistor audio power amplifier provide working point adjusting circuits and are instead fixed working point circuits.
Referring to FIG. 1, an example of known power amplifier circuits comprises a first NPN transistor Q1 and a second PNP transistor Q2 in the input stage, a third NPN transistor Q3 and a fourth PNP transistor Q4, where the base of Q1 is connected to first terminal A which is an input terminal and the base of Q2 is connected to second terminal B and where Q1 and Q3, Q2 and Q4 are respectively Darlington connected, a fifth NPN transistor Q5 and a sixth PNP transistor Q6 in the output stage, where Q3 and Q5, Q4 and Q6 are also respectively Darlington connected and where the collectors of Q1, Q3 and Q5 are connected to positive power supply B+ and the collectors of Q2, Q4 and Q6 are connected to negative power supply B-, a first resistor R1 which is connected between A and B+, a second resistor R2 which is connected between B and B-, a third resistor R3 which is connected between the bases of Q3 and Q4, a fourth resistor R4 which is connected between the emitters of Q3 and Q4, a fifth resistor R5 and a sixth resistor R6 which are connected between the respective emitters of Q5 and Q6 and the common third terminal C which is an output terminal, a seventh resistor R7, an eighth variable resistor R8, a ninth resistor R9, where R7, R8 and R9 are connected in series between A and B, a seventh NPN transistor Q7, a tenth resistor R10, where the base, collector and emitter of Q7 are connected to the variable end of R8, terminal A and terminal B through R10, respectively. The input stage differential amplifier circuit comprising Q1, Q2, R1 and R2 to which input signal is applied outputs two opposite phrase signals to the first complimentary circuit comprising Q3, Q4, R3 and R4 which form a current mirror circuit where the signal is converted into in-phase signal, therefore the second complimentary symmetry circuit comprising Q5, Q6, R5 and R6 in the output stage operates as a push-pull amplifier, and the bias circuit comprising R7, R8, R9, R10 and Q7 serves as a constant current means.
However, in this circuit the working point of the transistors can not be adjusted into the linear portion of their current-voltage characteristics so the distortion of the signal output to terminal C can not be eliminated and the energy efficiency of the output transistors is only about 70% and lost energy usually generates significant heat in the circuitry components.
The following prior art patent is also representative of known electronic power amplifier circuits:
17. U.S. Pat. No. 4,160,216 issued to Thornton on Jul. 3, 1979 for "Apparatus For Eliminating On-Off Transitional Gain Variations In Class AB, B And C Active Element Amplifier" (hereafter the "Thornton Patent").
The Thornton Patent discloses an improvement to an existing prior art amplifier circuit. After carefully comparing FIG. 4B to its corresponding prior art FIG. 4A, it is concluded that the four transistors 40A, 40B, 42A and 42B shown in FIG. 4B are existing transistors of the known prior art power amplifier circuit shown in FIG. 4A. The improvement provided by the Thornton Patent is the addition of the four diodes D40A, D40B, D44A and D44B. However, it does not include any transistors in its adjusting circuit, which substantially limits its ability.
The transistor-based adjusting circuit is desirable because it can adjust the working point of the transistors in the power amplifier circuit to a greater extent. It also can provide much less signal distortion and much more energy efficiency for power amplifiers. In addition it is very economical to have the means of making a Class B power amplifier work at the level of a Class A amplifier by simply using a working point adjusting circuit with only a few components in the power amplifier and without the high cost of the complicated construction of a Class A amplifier.
Another common feature of the prior art power amplifier circuits is that they typically have only one single output stage circuit which leads to a single output terminal. The single output terminal shown in the prior art FIG. 1 is marked as "C", or alternatively marked as "OUT" such as in FIG. 4B of the Thornton Patent. An output device, such as a loudspeaker, is connected to that output terminal. If the amplifier circuit is used to support two or more output devices, usually these output devices are connected to the single output terminal in a parallel relationship. However, using only one output stage circuit to support a multiplicity of output channels sometimes is not desirable, because it could result in power mismatch and sometimes even interference between the output devices such as loudspeakers. This is because output devices, even having the same model and power parameters, are not precisely identical due to the limitation in manufacturing. Therefore, for supporting multiple output channels or devices, it is desirable to have a multiple output stage circuit, such that each output channel or device is connected to an individual output stage circuit.